1. Field of the Invention
The present invention relates to a compressible layer for a printing blanket, made of vulcanized rubber and having a porous structure, which is incorporated into a printing blanket and a method of producing the same, and a printing blanket incorporating the compressible layer for a printing blanket.
2. Description of the Prior Art
In recent years, a so-called air-type printing blanket in which a compressible layer composed of an elastomer such as rubber and having a porous structure is provided under a surface printing layer similarly composed of an elastomer such as rubber has widely spread.
The air-type printing blanket is lower in compressive stress in a nip deformed portion produced by being pressed against a plate cylinder or the like, and is superior in impact absorbability because a variation in the compressive stress caused by the change in the amount of distortion is smaller, as compared with conventional solid-type printing blanket having no compressible layer. Therefore, the air-type printing blanket is superior in the effect of preventing impact produced at the time of feeding gears of a printing press, for example, from adversely affecting printing precision.
The solid-type printing blanket causes so-called bulge by stress concentrations on the surface printing layer in the nip deformed portion, which might result in inferior printing such as out of register, inferior paper feeding, double, or deformation of a dot pattern due to expansion in the circumferential direction. On the other hand, the air-type printing blanket can also prevent the inferior printing because the compressible layer has the function of lowering stress concentrations on the surface printing layer.
The following have been conventionally known as the above-mentioned compressible layer having a porous structure in the air-type printing blanket:
1 a compressible layer having an open cell structure, which is formed by a so-called leaching method for forming in a layered shape matrix rubber having common salt particles dispersed herein, vulcanizing the matrix rubber, and then extracting the common salt particles with warm water or the like, and
2 a compressible layer having a closed cell structure, which is formed by forming in a layered shape matrix rubber having a foaming agent which is decomposed by heating, for example, to emit gas dispersed therein, and foaming the foaming agent simultaneously with the vulcanization of the matrix rubber by heating at the time of the vulcanization.
The latter compressible layer having a closed cell structure has been paid attention to in recent years because it is superior in durability or the like to the former compressible layer.
The compressible layer having a closed cell structure makes it difficult to control the foaming of the foaming agent. Accordingly, the size of each of closed cells to be formed varies, or a plurality of cells communicate with each other in the foaming process to form a huge void. As a result, the cell structure of the compressible layer is non-uniform, so that compressibility varies, which adversely affects printing properties.
In order to solve this problem, it has been examined that hollow microspheres each having gas sealed in its spherical shell made of thermoplastic resin are used, to form a compressible layer having a closed cell structure. The hollow microspheres are supplied with their shapes and their particle diameters made nearly uniform. Therefore, it is considered that if matrix rubber having the hollow microspheres dispersed therein is formed in a layered shape, and is vulcanized, a compressible layer which has a uniform cell structure and do not vary in compressibility is obtained.
In the above-mentioned construction, however, it becomes clear that if the matrix rubber is vulcanized by applying heat and pressure under the conventional conditions using the conventional curing pan, for example, a compressible layer having sufficient compressibility is not obtained.
Specifically, the shells made of thermoplastic resin as described above are softened or melted by applying heat for a long time in vulcanizing the matrix rubber, and the hollow microspheres are deformed or collapsed by applying pressure in vulcanizing the matrix rubber. Therefore, a uniform closed cell structure with a sufficient porosity is not formed in the compressible layer, resulting in degraded compressibility of the compressible layer.
It has been examined that the matrix rubber is vulcanized without deforming or collapsing the hollow microspheres, and various proposals have been carried out.
For example, U.S. Pat. No. 4,770,928 (EP 0 342 286 B1) discloses a method of forming a compressible layer by holding matrix rubber, formed in a layered shape, having hollow microspheres dispersed therein over a long time period of 1 to 12 hours at a temperature of approximately 43 to 77.degree. C. which is significantly lower than the deforming temperature of the hollow microspheres to subject the matrix rubber to primary vulcanization without deforming or collapsing the hollow microspheres.
According to this method, in the subsequent step of laminating the compressible layer with the other layers constituting a printing blanket and finally subjecting an obtained laminate to secondary vulcanization to fabricate the printing blanket, even if a shell of each of the hollow microspheres is softened or melted or is lost upon being compatible with the matrix rubber by applying high temperature and high pressure as in the conventional example, the matrix rubber around the hollow microsphere which has already been vulcanized to some extent at the time of the previous primary vulcanization maintains the shape of a void where the hollow microsphere has existed. Therefore, it is possible to fabricate a printing blanket comprising a compressible layer having a high porosity and exhibiting uniform and superior compressibility.
In this method, however, a significantly long time period is required, as described above, to subject the matrix rubber to the primary vulcanization. Therefore, the productivity of the compressible layer and therefore, the productivity of the printing blanket are significantly lower than before. In order to subject the matrix rubber to the primary vulcanization at a significantly low temperature, as described above, a special vulcanization accelerator which is referred to as an ultra-accelerator must be generally used. Therefore, the fabrication cost of the printing blanket is high.
In the above-mentioned publication, a term "melting point of microcapsules (hollow microspheres)" is used with respect to the deforming temperature of the hollow microspheres. However, the thermoplastic resin such as a copolymer of acrylonitrile and vinylidene chloride, for example, which is exemplified in the publication has no definite melting point, as is well known. Accordingly, this term is unclear.
According to the examination by the inventors, the lowest temperature at which there occurs such a phenomenon that in heating the hollow microspheres, for example, for a predetermined time period (for example, approximately thirty minutes) under atmospheric pressure, for example, in an oven kept at a predetermined temperature, their shells are softened or melted during the above-mentioned time period,, so that many of the hollow microspheres are aggregated or integrated by fusing almost coincides with "melting point" in the above-mentioned publication or "melting temperature" in the following two prior arts. In the present specification, therefore, the heat resisting temperature of the hollow microspheres shall be represented by the deforming temperature of the hollow microspheres, as described above, and more specifically, the deforming temperature in a case where the hollow microspheres are heated under atmospheric pressure without applying pressure.
JP-A-3-244595 discloses, as an improvement of the above-mentioned publication, a method of bringing a layer of matrix rubber into contact with a drum which is warmed to a temperature which is as high as possible in temperatures lower than the deforming temperature of hollow microspheres (not more than approximately 100.degree. C. in the specification) or suspending the layer of the matrix rubber in an atmosphere which is warmed to the above-mentioned temperature to vulcanize the matrix rubber. It is disclosed that by employing such a method, the vulcanizing time can be shortened, as compared with that in a case where the layer of the matrix rubber is merely left to vulcanize the matrix rubber.
From the above-mentioned publication disclosing that the vulcanizing time which is one week at room temperature can be shortened, which is not definite because the degree to which the vulcanizing time can be shortened by employing this method is not specifically described, it is expected that a time period of one to tens of hours is required to vulcanize a compressible layer even if the vulcanizing time can be shortened. If so, it is considered that the problem that the productivity of the compressible layer and therefore, the productivity of the printing blanket is low remains unresolved.
In the above-mentioned method, the vulcanizing temperature is still a low temperature of not more than 100.degree. C. Therefore, the above-mentioned ultra-accelerator ("a strong material for vulcanizing rubber at a temperature of not more than 100.degree. C." described in the above-mentioned publication is a ultra-accelerator) must be used as a vulcanization accelerator. Therefore, the problem that the fabrication cost of the printing blanket which is based on the use of the ultra-accelerator is high remains unresolved.
In order to solve the problems, it has been proposed in WO 93/18911 A1 that hollow microspheres having high heat resistance whose deforming temperature is not less than 135.degree. C. are used, and the primary vulcanizing temperature of matrix rubber is set to a temperature lower than the deforming temperature of the hollow microspheres but slightly higher than before, for example, approximately 80 to 150.degree. C.
As the effect, the publication discloses that the primary vulcanizing time can be reduced to one to six hours, and the compressible layer can be formed using a general-purpose vulcanization accelerator without using the above-mentioned special ultra-accelerator.
Even in the above-mentioned method, however, the vulcanizing time is still a long time of not less than one hour. Therefore, the quantity of heat applied to the hollow microspheres during the vulcanization is large, and the primary vulcanizing temperature of the matrix rubber and the deforming temperature of the hollow microspheres are close to each other. Therefore, it has been made clear by the examination by the inventors that at the time of actual vulcanization, vulcanization reaction of the matrix rubber, the deformation or collapse of the hollow microspheres by softening or melting their shells progress almost simultaneously and competitively.
In a case where a compressible layer is subjected to primary vulcanization by a method in which the quantity of heat received by a layer of matrix rubber having hollow microspheres dispersed therein which forms the basis of the compressible layer, for example, may greatly vary as in a case where the layer of the matrix rubber, together with one base fabric for supporting the layer, is wound in a roll shape, and is vulcanized in a curing pan, a portion where the hollow microspheres have already been deformed or collapsed, although the matrix rubber around the hollow microspheres is sufficiently vulcanized or a portion where the matrix rubber around the hollow microopheres is insufficiently vulcanized, although the hollow microspheres are not deformed or collapsed, and the hollow microspheres are deformed or collapsed at the time of secondary vulcanization which is the subsequent step occur in the compressible layer. As a result, the porosity of the compressible layer in the printing blanket greatly decreases and varies from place to place. Therefore, it has been clear that the compressibility may be degraded.